# include <bits/stdc++.h>
using namespace std;
#define ll long long
signed main(){
    
    string s; cin>>s;
    vector<ll>freq(26);
    for(auto it:s) freq[it-'a']++;
    bool f=0;
    for(ll i=0;i<26;i++)
    {
      if(freq[i]!=0) {cout<<char(i+'a'); f=1;}
      if(f) break;
    }
    if(!f) cout<<-1<<endl;
    return 0;
}